May 9, 2008

Farmers are reporting that glyphosate used alone does not work as well today as it did several years ago. Nebraska fields are experiencing a slow shift in weed species. In the last three years, university weed extension specialists have been receiving phone calls and complaints about glyphosate failing to control certain weed species, including some "new weeds." These species include: marestail (horseweed), morning-glory (common and ivyleaf), wild buckwheat, Pennsylvania smartweed, lady=s thumb, venice mallow, yellow sweetclover, field bindweed, waterhemp, kochia, Russian thistle, primrose species and volunteer Roundup-Ready corn.

If these weeds are not controlled, their seeds will be a major problem in the future, especially in systems not using tillage for weed control. Weed population shifts to more tolerant weeds already are resulting in increased weed control costs.

The purpose of this article is to summarize data from UNL studies at Concord and North Platte in 2004 testing six soil-applied herbicides for control of the weed species listed above. The six broadleaf herbicides were: Authority 75DG (5oz/ac), Sencor 75DF (8 oz/ac), Canopy XL 56DG (6.5 oz/ac), Commit 3ME (1.5 pt/ac), Pursuit Plus 2.9EC (2.5 pt/ac), Scepter 70DG (2.8 oz/ac) and Steel 2.6 EC (3 pt/ac). These herbicides were applied to the soil after planting weed seeds.

The level of weed control at 40 days after planting varied by the weed species and herbicide. For example, Sencor provided excellent control (100%) of kochia, velveltleaf and Venice mallow but less control (37%) of ivy leaf morningglory (Table 1).

The results indicate a potential to effectively control most of those weed species with pre-emergence herbicides applied to the soil after soybean planting. Soil-applied herbicides also would provide additional mode of action for weed control, reducing a chance for weed resistance. Soil-applied herbicides also provide a longer "comfort zone" for weed control early in the season by delaying the critical time for weed removal and reducing the need for multiple glyphosate applications later in the season.

Integrating weed control tools is not a new thing, but has been less common since the introduction of Roundup Ready crops. Changing modes of actions in your herbicide program is a basic aspect of an Integrated Weed Management (IWM) program, especially when used to combat weed resistance and tolerance issues. When Roundup-Ready technology is used as part of an integrated weed management system, this technology can be preserved and its overuse can be avoided. IWM programs are based on a few general rules that can be used on any farm:

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• use of agronomic practices that limit the introduction and spread of weeds (preventing weed problems before they start);
• help the crop compete with weeds; and
• use practices that do not allow weeds to adapt.

Combining agronomic practices based on these rules allows you to design a program for any field, changing and tailoring annually as your operation makes change. The goal is to manage weeds below an economic level, not eliminate them. . For more details on applying integrated weed management to your operation, see the Guide for Weed Management in Nebraska.

The concepts of IWM become even more important with the increased use of other Roundup-Ready crops (eg. Roundup-Ready corn, Roundup-Ready alfalfa). It is easy to fall into a trap of overusing glyphosate when one glyphosate-tolerant crop is grown after another, but proper use of this technology as part of an integrated weed management program, is key to preserving this technology while avoiding overuse.

Stevan Knezevic
Extension Weeds Specialist
Haskell Ag Lab, Concord, Northeast REC